The present invention relates generally to magnetic resonance imaging systems, such as those used in medical diagnostics. More particularly, the invention relates to a technique for reducing acoustic noise in magnetic resonance imaging systems.
Over the last decades, magnetic resonance imaging (MRI) systems have found widespread use, particularly in medical diagnostics applications. MRI systems make use of the physical influence of magnetic fields on gyromagnetic material within a subject to be imaged. In general, by imposing external magnetic fields on the subject, nuclei having characteristic precession frequencies can be made to orient themselves and to spin in known ways, to produce detectable signals.
In current MRI systems, the particular magnetic fields imposed on the subject can be manipulated so as to produce high-quality, reliable images for use in medical diagnostics. In conventional MRI systems, for example, gradient fields are produced which define a desired slice through the subject, and which encode the positions of the materials of interest through the selected slice in accordance with their location. The gradient fields are produced in the presence of a primary magnetic field, generally oriented longitudinally with respect to the subject. After imposition of a radio frequency pulse which produces a transverse moment in the gyromagnetic material, echo signals from the material can be sensed and processed to identify the intensity of the response at the various locations in the slice. After such data processing, an image can be reconstructed based upon the acquired and processed data.
Numerous improvements have been made in recent years, both to MRI imaging systems and to the techniques employed to produce resulting images. Many of these techniques permit reductions in image artifacts by virtue of novel pulse sequences implemented through gradient and RF coil structures. Improvements in the physical structures of MRI systems have also improved image quality and reduced the occurrence of artifacts. In many cases, the physical system improvements have worked in conjunction with software enhancements to enable much faster and higher quality imaging. Examination sequences which required minutes and seconds in the past may now be performed in a fraction of that time.
Although MRI systems have improved dramatically, they remain plagued by certain characteristic problems. A particular problem in the systems is the acoustic noise generated by application of the pulse sequences to the coil structures needed to produce the desired images. As improved coil structures and pulse sequences have been improved to obtain the benefits discussed above, the resulting acoustic noise has often become exacerbated due, for example, to higher currents and higher slew rates, and the rapidity of execution of pulse sequences. While many attempts have been made to locate sources of acoustic noise, and to reduce the noise, such as through changing the characteristic frequencies and amplitudes of vibration of mechanical components, and so forth, acoustic noise remains a serious problem in many systems. While the examination durations are often shorter through the use of new pulse sequences and processing techniques, many patients still find MRI examinations discomforting due to the acoustic noise associated with the examination sequences.
There is a need, at present, for techniques capable of reducing acoustic noise of MRI systems, at least for certain examination sequences or certain patients. There is a particular need for a simple and straightforward technique for selectively reducing acoustic noise in state-of-the-art MRI systems which does not impair the improved functionality of the systems for most pulse sequences, but which accommodates the needs of more sensitive patients. Moreover, there is a need for a technique which does not require extensive reconfiguration of the system by an operator when a sensitive patient is present.
The present invention provides a technique for reducing acoustic noise in MRI systems designed to respond to these needs. The technique may be implemented in a wide variety of MRI systems, including existing and new systems of any type or manufacture. The technique may also be applied with any type of pulse sequence, and is particularly well suited to pulse sequences apt to generate significant acoustic noise. The technique may be used in parallel with existing, conventional MRI control circuitry, to permit pulse sequences to be implemented for examinations on a normal basis, and to permit quieter sequences to be implemented when needed.
The reduced noise mode of operation may call upon a set of configuration parameters which is different from the normal mode parameters for the same pulse sequences. Alternatively, the quiet mode pulse sequences may be derived from the normal mode pulse sequences, such as through imposition of gradient amplitude limits, slew rate limits, coefficients or multipliers to rescale parameters, and so forth. The quiet mode of operation may be a distinct, xe2x80x9con-offxe2x80x9d selection which is made by toggling a switch, presented, for example, as a virtual button in a graphical user interface at an operator station. Alternatively, the quiet mode of operation may provide an adjustable range of settings varying from the normal mode, such that the operator may adjust the acoustic noise indirectly through the use of a virtual slide in a graphical user interface. Other types of interface may, of course, be employed. In either case, the quiet mode of operation results in reduced acoustical noise by modification of the pulse sequence for the desired examination. The resulting images may be similar to those of the normal mode images, but require slightly longer acquisition times. In general, however, image quality may be balanced against comfort of the patient, where needed, to provide the best available image for individual patients, depending upon their sensibilities.